1. Field of the Invention
The present invention relates to photography, and, in particular, to an auto focus adjustment system and method used to control a photographic optical system of a camera.
2. Description of Related Art
An auto focus adjustment system is known in which the focus adjustment condition of the photographic optical system or the defocus amount, which is the distance that the image plane of the photographic optical system deviates from the predicted focus plane, is detected for each one of several focus detection regions arranged within the shooting field. A final defocus amount is determined from among the plurality of defocus amounts detected. Accordingly, the focus condition (i.e., the condition in which the camera is focussed upon the desired subject) is obtained by driving the photographic optical system according to this final defocus amount.
The following two methods are known for determining the final defocus amount from among a plurality of defocus amounts:
(1) Current status priority mode
In this mode, from a plurality of defocus amounts, the one with the smallest absolute value is selected as the final defocus amount. For example, suppose defocus amounts D1, D2, and D3 are obtained for the three focus detection regions shown in FIG. 16.
In FIG. 16, the horizontal axis represents the position of the focus detection region and the vertical axis represents the defocus amount. In the present specification, the defocus amount, in the case in which the image plane of the photographic optical system is formed on the photographic optical system side of the predicted focus plane, is defined as positive. In the case in which the image plane is formed on the opposite side, the defocus amount is defined as negative. Therefore, a negative defocus amount represents the minimum defocus amount of the closest side.
In the current status priority mode, a defocus amount D1 having the smallest absolute value from among the three defocus amounts is selected.
(2) Closest side priority mode
In this mode, a defocus amount corresponding to the closest subject from among a plurality of defocus amounts is selected as the final defocus amount.
For example, suppose defocus amounts D1, D2, and D3 are obtained in each of three focus detection regions shown in FIG. 17. In the closest side priority mode, the defocus amount D3 corresponding to the closest subject 3 is selected as the final defocus amount.
However, the following problems arise in a conventional auto focus adjustment system in which the final defocus amount is determined according to the methods described above:
(1) In the current status priority mode, suppose that focus detection is executed by an auto focus adjustment system of a camera that establishes three focus detection regions FL, FC, and FR within the shooting field as shown in FIG. 3, for example, in three subject field regions L, C, and R, as shown in FIG. 18. Further, suppose that subject 1, subject 2 and subject 3 are located within these three subject field regions L, C, and R, respectively, at the shooting length R0. In such a shooting condition, the auto focus adjustment system of a camera adjusts the focus of the photographic optical system for a position at a distance equal to shooting length R0.
In this instance, suppose that subject 2 approaches the camera and reaches a shooting length R1 after the photographic optical system has been adjusted to a distance equal to the shooting length R0. In this case, subjects 1 and 3 remain in regions L and R, respectively, at a distance equal to the shooting length R0. Accordingly, in the current status priority mode, the defocus amount with the smallest absolute value from among a plurality of defocus amounts (i.e., defocus amounts corresponding to subject 1 or subject 3 in this case) is selected as the final defocus amount and the focus condition for subject 1 or subject 3 is maintained as the result of adjusting the focus according to the final defocus amount.
In general, when some of the subjects approach the camera, the subject closest to the camera is the one on which the photographer desires to focus. However, in the current status priority mode, subject 2, which is closest to the camera, is out of focus and blurred.
(2) In the closest distance priority mode, suppose that the focus detection routine is executed by an auto focus adjustment system of a camera that establishes three focus detection regions FL, FC, and FR within the shooting field as shown in FIG. 3, for example, in three subject field regions L, C, and R, as shown in FIG. 19. Further, suppose that subject 1, subject 2, and subject 3 are located within these three subject field regions, L, C, and R, respectively, at a distance equal to the shooting length R0. In such a condition, the auto focus adjustment system of the camera adjusts the focus of the photographic optical system to a position with a distance equal to the shooting length R0.
Suppose that an undesired object crosses in front of the camera at a distance R2 after the photographic optical system is adjusted to a distance equal to the shooting distance R0. The size of this undesired object is assumed to be smaller than the total size of focus detection regions FL, FC, and FR in FIG. 3. In the closest distance priority mode, the smallest defocus amount from among a multiplicity of defocus amounts (i.e., a defocus amount corresponding to the subject at the closest distance) is selected as the final defocus amount. Thus, the camera is focussed on the undesired object at the closest distance as a result of adjusting the focus according to the final defocus amount.
Thus, when the undesired object crosses in front of the plurality of subjects, the intended subjects become out of focus and blurred despite the photographer's desire to maintain the subjects in focus.